So according to the wiki, you put an object in another objects container, and each becomes a container for the other (very futuramaish). Then it goes on to say that any instance of one, will include the other, and anything done to one will effect the other. So using it's example of a tank and a turret. Everytime a tank instance is spawned, you'd get the turret in the right place.

But it doesn't, you just get the part that's spawned, not both like it says you should. It suggests looking at the RTS example, but that's broken and wont run in the current version.

So what am I doing wrong. Or are containers broken right now? Because they're certainly not doing what the wiki says they should be doing, in fact they don't appear to do anything. Bug?

It still creates the instance but you need to tell it where to put it. Putting two things in a container will make them spawn together and when one is picked so is the other. you would need an always event setting the position of the other object.